KR102017553B1 - Mold steel for long life cycle die casting having high hardenability and superior nitriding property - Google Patents

Abstract

The present invention relates to a hot-work mold steel comprising: 0.37 to 0.46 wt% of carbon (C), 0.25 to 0.5 wt% of silicon (Si), 0.36 to 0.56 wt% of manganese (Mn), 2.0 to 5.0 wt% of chromium (Cr), 1.4 to 2.6 wt% of molybdenum (Mo), 0.4 to 0.8 wt% of vanadium (V), 0.0007 to 0.004 wt% of boron (B), 0.002 to 0.022 wt% of aluminum (Al), 0.001 to 0.09 wt% of titanium (Ti), and the remainder of iron (Fe) and inevitable impurities, and a manufacturing method therefor. The hot-work mold steel exhibits excellent thermal conductivity, hardenability, durability, and nitriding properties, thereby increasing resistance to heat check and melt-out. When a die-casting mold is manufactured by using such a hot-work mold steel, the die-casting mold has improved thermal conductivity regardless of mold size. Further, the life of the mold is extended, and parts manufactured by using the mold can have improved surface quality.

Description

Hot die steel for long life die casting with excellent hardenability and nitriding properties and its manufacturing method {Mold steel for long life cycle die casting having high hardenability and superior nitriding property}

The present invention relates to a hot die steel for die-casting and a method of manufacturing the same, which are excellent in hardenability and nitriding properties, and more particularly, hot hardening and nitriding properties that can be applied when producing automobile parts. The present invention relates to a mold steel and a method of manufacturing the same.

Hot die steels include carbon, chromium, silicon, nickel, molybdenum, manganese, vanadium and cobalt in addition to iron as alloying elements, and have excellent mechanical properties even at high temperatures by including these alloying elements, and thus have excellent mechanical properties at high processing temperatures. It is used in the manufacture of extrusion dies, forging dies and die casting molds where strength properties are required.

Hot die steels and steel objects produced using them, for example dies such as die casting dies, when introduced into the technical process, must have a uniform temperature on the entire surface of the mold so that the molded parts have a uniform quality. High heat conductivity is required because the heat must be released sufficiently, and in addition, high heat resistance is required.

In addition, since the resistance to heat check and melting loss of the hot die steel is directly related to the life of the hot die, surface treatment including nitriding is performed to maximize these characteristics. Nitrogen injection depth on the mold surface and the degree of formation of the nitrogenous compound formation layer through nitriding are directly related to the chemical composition constituting the mold, so that the heat check and dissolution resistance due to nitriding are also directly affected by the chemical composition.

Important properties of hot die steel other than those mentioned above include the hardenability. The higher the hardenability, the more homogeneous and tougher structure can be obtained over the wider range under the same heat treatment conditions. When the hot mold steel has a high hardenability, it is possible to manufacture a stronger mold, and also to manufacture a mold having various sizes.

Recently, the share of lightweight non-ferrous metals is increasing due to the trend of eco-friendly and high fuel consumption in the automotive industry, and the demand for hot die steel for die casting to form them is increasing, but the domestic hot die steel market for die casting used in the domestic market, such as Hitachi, It is urgent to localize hot die steel for die casting as it is occupied by advanced companies, and it is difficult to give sufficient hardenability and nitriding properties to long die steel for long life die casting with existing technology. Development is required.

Registered Patent No. 10-0834535 (2008.05.27 registration)

The present invention is to provide a hot die steel that can produce a long life die casting excellent in hardenability and nitriding properties by optimizing the composition of the components forming the hot die steel and its manufacturing conditions.

One embodiment of the present invention for achieving the above object is 0.37 to 0.46% by weight of carbon (C), 0.25 to 0.5% by weight of silicon (Si), 0.36 to 0.56 weight of manganese (Mn) %, Chromium (Cr) 2.0 to 5.0 wt%, Molybdenum (Mo) 1.4 to 2.6 wt%, Vanadium (V) 0.4 to 0.8 wt%, Boron (B) 0.0007 to 0.004 wt%, Aluminum (Al) 0.002 to 0.022 wt% %, 0.001 to 0.09% by weight of titanium (Ti) and iron (Fe) and inevitable impurities, wherein the weight% value of each element contained is characterized in that both satisfy the following formula (1) and (2) It relates to mold steel.

The hot mold steel may further include 0.001 to 0.007 wt% of tungsten (W), and may further include 0.001 to 0.025 wt% of niobium (Nb), and cobalt (Co) 0.005 to 0.022 wt%. It may further include a.

In addition, the hot die steel may be a die-casting die steel (die-casting) obtained through the quenching and tempering step, the quenching step is carried out in a temperature range of 1000 ~ 1040 ℃, the tempering step of 520 ~ 640 ℃ It can be carried out in a temperature range.

Meanwhile, another embodiment of the present invention is 0.37 to 0.46% by weight of carbon (C), 0.25 to 0.5% by weight of silicon (Si), 0.36 to 0.56% by weight of manganese (Mn), and chromium (Cr) 2.0 to 5.0 wt%, molybdenum (Mo) 1.4-2.6 wt%, vanadium (V) 0.4-0.8 wt%, boron (B) 0.0007-0.004 wt%, aluminum (Al) 0.002-0.022 wt%, titanium (Ti) 0.001- A forging step of heat-treating the hot die ingot, which comprises 0.09% by weight, the remainder including iron (Fe) and unavoidable impurities; Quenching step of heating and then cooling the mold material produced in the forging step; And a tempering step of heat-treating the mold material quenched in the quenching step at a temperature range of 520 to 640 ° C., wherein the weight% value of each element included includes all of the following Equations (1) and (2): It relates to a method for producing a hot die steel to be satisfied.

At this time, the heat treatment of the forging step is carried out at a temperature range of 850 ~ 1300 ℃, it is preferable that the forging ratio of 4.5S or more.

In addition, a spheroidization heat treatment step may be further included between the forging step and the quenching step. The spheroidization heat treatment step may be performed at a temperature range of 840 to 900 ° C.

The heating process of the quenching step may be carried out at a temperature range of 1000 ~ 1040 ℃, the cooling process may be performed at a cooling rate of 0.2 ~ 3.0 ℃ / s, through this cooling process temperature range of 80 ~ 100 ℃ It is preferable to cool to.

The tempering step, the first tempering step of heat-treating the quenched mold material for 2 hours to 6 hours at a temperature range of 540 ~ 630 ℃; And a second tempering step of heat treating the mold material for 2 hours to 6 hours in a temperature range of 540 to 620 ° C. In addition, the third tempering step of heat-treating the hot mold steel obtained through the second tempering step for 2 hours to 6 hours at a temperature range of 540 to 610 ℃; may be further included.

The nitriding heat treatment step may be further performed after the tempering step, and the nitriding heat treatment step may be performed by any one of a pure nitriding method, a gas nitriding method, a soft nitriding method, an ion nitriding method, and a immersion nitriding method.

The hot mold ingot may further comprise 0.001 to 0.007% by weight of tungsten (W), and may further include 0.001 to 0.025% by weight of niobium (Nb), and cobalt (Co) 0.005 to 0.022% by weight. It may further include a.

Since the hot die steel of the present invention has excellent hardenability, durability, and nitriding properties, and excellent resistance to heat check and melting loss, it is possible to manufacture molds of various sizes ranging from small to large, and also greatly improve the life of the mold. Can be.

In addition, the hot mold steel provided by the present invention has a high thermal conductivity at high temperature, so that the surface quality of the parts manufactured using the same may be improved.

1 is a graph showing the relationship between the cooling rate according to the nitrogen pressure and the thickness of the hot mold steel during the cooling step of the quenching step, a represents the cooling rate of the center portion, b represents the cooling rate of the surface portion.
2 is a graph showing the thermal conductivity represented by the formula (1) according to the content of carbon and boron.
3 is a graph showing the critical cooling rate expressed by the formula (2) according to the content of carbon and boron.
4 is a graph showing a continuous cooling transformation diagram of Experimental Example 2. FIG.
5 is a photograph showing a result of measuring a Debye ring according to the quenching temperature of Experimental Example 4 using an X-ray diffractometer.
6 is an optical picture showing a change in the structure according to the quenching temperature of Experimental Example 4.
Figure 7 is a graph showing the results of measuring the hardness change according to the quenching temperature of Experimental Example 4.
8 is a graph showing the results of measuring the hardness change according to the tempering temperature of Experimental Example 5.
9 is a graph showing the results of measuring the change in hardness according to the tempering time of Experimental Example 6.
10 is a graph showing the hardness according to the distance from the surface of the mold steel after nitriding of Experimental Example 7.
11 is a graph showing a heat check result according to the thermal conductivity of Experimental Example 8.
12 is a photograph showing the melting loss test result of Experimental Example 9. FIG.
13 is a graph showing the depth of melting according to the molybdenum content of Experimental Example 9.

Hereinafter, preferred embodiments of the present invention will be described. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below. In addition, the embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Throughout this specification, when a part is said to "include" a certain component, it means that it can further include other components, without excluding other components unless otherwise stated.

In addition, in this specification, all percentages defined by mass are the same as those defined by weight, respectively.

Hot die steel according to an embodiment of the present invention, carbon (C), silicon (Si), manganese (Mn), chromium (Cr), molybdenum (Mo), vanadium (V), boron (B) as an essential element , Aluminum (Al) and titanium (Ti), the remainder is composed of iron (Fe), microelements and inevitable impurities.

Specifically, 0.37 to 0.46% by weight of carbon (C), 0.25 to 0.5% by weight of silicon (Si), 0.36 to 0.56% by weight of manganese (Mn), 2.0 to 5.0% by weight of chromium (Cr), and molybdenum (Mo) ) 1.4 ~ 2.6 wt%, Vanadium (V) 0.4 ~ 0.8 wt%, Boron (B) 0.0007 ~ 0.004 wt%, Aluminum (Al) 0.002 ~ 0.022 wt%, Titanium (Ti) 0.001 ~ 0.09 wt%, the rest is iron When the content value of each element comprising (Fe) and unavoidable impurities and forming the hot die steel is substituted into the following formula (1), the value is 30.5 or more, and substituted into the following formula (2), The value satisfies 0.35 or less.

The hot mold steel may further include 0.001 to 0.007 wt% of tungsten (W), may further include 0.001 to 0.025 wt% of niobium (Nb), and may further include 0.005 to 0.022 wt% of cobalt (Co). It may be. In addition, the inevitable impurities may include phosphorus (P), sulfur (S), nitrogen (N) and oxygen (O), and may also include other materials.

Hereinafter, the reason for limiting the components constituting the hot die steel and its composition range as described above will be described.

Carbon (C)

Carbon is an essential element necessary to control the strength of steel.In this alloy system, alloy carbide forms alloy carbides, which affects grain refinement, and secondary hardening affects durability, such as high temperature yield strength, in particular, heat treatment in the quenching stage. It is one of the elements that effectively improves the hardenability of the alloy.

If the carbon content is less than 0.37% by weight, the hardness and strength is lowered, the hardenability is lowered to obtain a uniform cross-sectional hardness, and when the carbon content exceeds 0.46% by weight, crystallized carbides are formed to worsen the fatigue strength and the impact value. In addition to this, the martensite transformation temperature is significantly lowered to increase the amount of residual austenite after the heat treatment in the quenching step, which may lower the dimensional change and toughness of the steel after the final heat treatment.

Therefore, it is preferable that the hot die steel of the present invention contains 0.37 to 0.46% by weight of carbon in order to impart excellent hardness and strength to the hot die steel while not causing other physical property deterioration.

silicon( Si )

Silicon is an element that suppresses the decomposition of residual austenite produced by heat treatment in the quenching step into needle-like cementite during heat treatment in the tempering step. In this alloy system, needle-like cementite is a heat check generated in the die casting process. Since the resistance to the metal can be drastically reduced, by including the silicon in the mold steel in an appropriate amount, the toughness of the steel can be improved and the heat check resistance can be improved.

In addition, since silicon contributes to the improvement of hardenability but may lower the thermal conductivity, it is preferable to be used within a range that does not significantly affect the thermal conductivity. When the silicon content is less than 0.25% by weight, the heat check is performed. Since the effect of improving the resistance against is negligible and the silicon content exceeds 0.5% by weight, the thermal conductivity is lowered. Therefore, the hot die steel of the present invention preferably contains 0.25 to 0.5% by weight of silicon.

Manganese (Mn)

Manganese is an element that improves hardenability and causes solid solution reinforcement. If the content of manganese is less than 0.36% by weight, it is difficult to obtain hardening effect and solid solution strengthening effect, and the content of manganese is 0.56% by weight. Since the thermal conductivity is significantly reduced when it is exceeded, it is preferable to add 0.36 to 0.56% by weight based on the total weight parts in order to secure the efficacy of manganese while preventing the decrease in thermal conductivity.

chrome( Cr )

Chromium not only improves hardenability, but also produces complex carbides to improve hardness, strength and softening resistance and wear resistance in the tempering step, and forms a nitrogen compound in the nitriding step to improve surface hardness. The content of chromium is preferably 2.0 to 5.0% by weight based on the total weight, it is difficult to expect the effect of improving the hardenability when included in less than the weight range, the problem of reducing the thermal conductivity when included in excess of the weight range Since it occurs, it is preferable to be included within the above weight range.

molybdenum( Mo )

Molybdenum forms carbides such as molybdenum carbides to increase high temperature hardness and strength, and improves high temperature strength by causing secondary curing at high temperatures during tempering, as well as bonding with phosphorus (P) present at grain boundaries. Since it is an element which prevents temper brittleness due to the poet and does not affect the thermal conductivity, it is preferable that 1.2 wt% or more of the total weight is included.

More preferably, the molybdenum content is less than 1.4 wt%, and the temper embrittlement suppression force due to phosphorus is reduced, and the secondary hardening does not occur sufficiently to reduce hardness and strength at high temperature. This is because when the content of molybdenum exceeds 2.6% by weight, the effect of improving strength and inhibiting temper brittleness by molybdenum may be reduced.

Tungsten (W)

Tungsten is an element that can be selectively added to increase the strength of the hot die steel, which increases the strength of the hot die steel by inducing precipitation hardening of carbides and, like molybdenum, can provide the effect of secondary hardening.

Tungsten may be included in an amount of 0.001 to 0.007% by weight based on the total weight, and when included in the amount below the weight range, the effect of improving the strength of the mold steel is insignificant, and when included in the weight range, the tungsten may decrease the thermal conductivity of the mold steel. It is preferable to be included within the above weight range because there is.

titanium( Ti )

Titanium has a low solubility in austenite, so that a strong precipitated phase may be produced. In this alloy system, titanium has an effect of imparting a structure refining effect, but due to the bonding strength of titanium with carbon, the carbon content in the austenite matrix is reduced and hot. Since there is a side effect of lowering the hardenability of the mold steel, and the degree of hardenability is lower than vanadium, it is preferable to be included to such an extent that the hardenability is not significantly reduced while showing sufficient structure refining effect. Specifically, since titanium is 0.001% to 0.09% by weight based on the total parts by weight, it is possible to minimize the side effects while obtaining the above effects when included.

Vanadium (V)

Vanadium is an element that substitutes and solidifies with iron to increase tensile strength, make insoluble carbides, increase high temperature hardness, and increase temper brittle resistance, and in particular, produces a fine precipitated phase at high temperature to inhibit austenite grain growth. Has Vanadium, together with titanium and niobium, forms strong alloy carbides to refine grains, but it is less likely to be crystallized due to weaker bonding strength with carbon than titanium and niobium, and causes less carbon content in the austenitic matrix. In comparison, the influence on the thermal conductivity and the hardenability decrease is relatively small.

If the content of vanadium is less than 0.4% by weight relative to the total weight of the hot die steel, it is difficult to sufficiently obtain a grain refining effect, and if it exceeds 0.8% by weight, crystallized carbides may be formed. It is preferable.

Niobium ( Nb )

Niobium, like titanium, has a low solubility in austenite to produce a strong precipitated phase, and is an element that gives a structure refinement effect. In addition, the bonding strength with carbon is strong to reduce the carbon content in the austenite matrix to weaken the hardenability of the hot mold steel, the degree is larger than vanadium, an alloying element that plays a role of other grain refinement.

When the content of niobium is less than 0.001% by weight based on the total weight, it is difficult to obtain a micronized structure due to niobium, and when it exceeds 0.025% by weight, the hardenability of the hot mold steel may be lowered, so it may be 0.001 to 0.025. It is preferably included in the weight percent.

Boron (B)

Boron can greatly improve the hardenability due to grain boundary segregation even with the addition of a very small amount. However, when boron is contained in an amount less than 0.0007% by weight based on the total weight, it is difficult to obtain sufficient hardenability improvement effect, and 0.004% by weight When included in excess, since the degree of improvement in hardenability is insignificant compared to the additionally added amount, and economic loss occurs, it is preferable to include 0.0007 to 0.004% by weight based on the total weight of the hot mold steel.

Cobalt (Co)

Cobalt dissolves only in the base to increase the solubility of carbon, and it is an element that not only has a large amount of carbides in the base, but also has a strengthening effect due to the strengthening of solid solution. It is effective when added to 0.005 to 0.022% by weight based on the total weight. It may be obtained, if included in the composition outside the weight range it is not possible to obtain the effect, or it is preferable to include within the weight range because it is difficult to expect additional effects according to the excess amount.

Phosphorus (P)

Phosphorus contributes to the increase in strength of the hot die steel in part, but if it exceeds 0.007% by weight, it is preferable to be included as 0.007% by weight or less, and more preferably 0.005 to 0.006% by weight, because there is a problem of deteriorating weldability. have.

Sulfur (S)

Since sulfur combines with manganese to cause toughness degradation and hot cracking, sulfur is preferably included at 0.003% by weight or less.

Nitrogen (N)

Nitrogen is an impurity contained in steelmaking. In order to obtain grain boundary segregation of boron, nitrogen can reduce the physical properties of the hot die steel by forming boron nitrogen, but there is a solid solution effect. It is preferably included in 0.005 to 0.06% by weight.

Aluminum (Al)

Aluminum is an element that is added to offset the side effects caused by nitrogen boron because it has a stronger bonding force with nitrogen than boron. When aluminum is contained in an amount of 0.002 to 0.022% by weight, aluminum is used to remove only a small amount of nitrogen dissolved. It may be used and is added within the weight range because it can lower the physical properties of the mold steel outside the above range.

Except for the above-described components, the hot die steel of the present invention, the rest is substantially made of iron (Fe), and the rest is made of iron (Fe), unless the action of the effect of the present invention, Including inevitable impurities, including other trace elements also means that can be included within the scope of the present invention.

As described above, when the content values of carbon, silicon, manganese, chromium, vanadium, boron and titanium constituting the hot die steel of the present invention are substituted into the following formula (1), the value is 30.5 W / mK or more. Satisfies. When the die casting process is performed using a mold made of hot mold steel having high thermal conductivity, the temperature difference between the inside and outside of the mold is reduced, thereby improving the resistance to the heat check of the mold, thereby extending the life of the mold. The surface quality of the product manufactured by such a mold can be improved.

Equation (1) generates 161 alloys in the alloying element content range consisting of eight alloying elements using the Box-Behnken method in the experimental design method, and simulated software for each alloy produced (J-Mat Pro). The thermal conductivity at 400 ℃ is calculated by using) and then the statistically significant coefficients are derived from the complete quadratic model. In particular, since the influence of carbon and boron is shown to be large in thermal conductivity, the present invention improves the thermal conductivity of the hot die steel of the present invention by limiting the content of carbon and boron as described above.

Equation (1) is the calculated thermal conductivity of the alloy at 400 ℃, slightly larger than the value of the thermal conductivity obtained in the actual experiment, 400 ℃ is 400 ℃ because it is a representative temperature obtained by the molten metal during the die casting process of hot die steel A relation based on thermal conductivity at was used.

The weight% value of the elements constituting the hot die steel of the present invention satisfies Equation (1), and the minimum value of 30.5 W / mK is set in Equation (1), and the weight% of each element of the hot die steel of the present invention. When the value calculated by substituting the value on the left side of Equation (1) is 30.5W / mK or more, the length of the heat crack is significantly reduced, which can be confirmed in Experimental Example 8 which will be described later.

In this way, the mold made of hot mold steel having high thermal conductivity can alleviate the nonuniformity of temperature of each part of the mold to reduce the shrinkage and torsion of the molded article, thereby making a molded article having a uniform quality as well as the quality of the molded article. It can also be improved. In addition, the frequency and degree of heat check crack occurrence due to the temperature difference according to the mold part may be reduced, thereby significantly improving the life of the mold.

Meanwhile, the weight% values of carbon, silicon, manganese, chromium, vanadium, boron, molybdenum and titanium constituting the hot die steel of the present invention satisfy the formula (2), and the hot die steel having such a low critical cooling rate is It can have excellent hardenability.

Equation (2) is derived from the pearlite transformation or bainite during cooling after deriving 161 continuous cooling diagrams of the same alloy used for deriving Equation (1) using computer simulation software (J-Mat Pro). This formula is designed to derive statistically significant coefficients using a quadratic model after obtaining the minimum cooling rate (critical cooling rate) without occurrence of (bainite) transformation.

The critical cooling rate is a value that affects the hardenability of the steel. When cooling faster than the critical cooling rate in the quenching stage, a complete martensite structure can be obtained from the inside of the steel.As the critical cooling rate becomes smaller, the complete martensite is given at a given cooling rate. Smaller critical cooling rates are advantageous for product production because the size of the product from which the tissue can be made increases and the thickness of the surface with complete martensite tissue increases.

As confirmed by Equation (2), since the influence of carbon and boron on the critical cooling rate is large, the present invention is limited to the content of carbon and boron as described above, and has a high hardenability by reducing the critical cooling rate. Can provide river

As can be seen from a and b of Figure 1, both the central portion and the surface portion shows a lower cooling rate as the thickness of the hot die steel becomes thicker. If the cooling rate is lower than the critical cooling rate, the hardenability is lowered and sufficient mechanical properties cannot be given to the steel, so it can be seen that it is necessary to secure a low critical cooling rate.

In general, the thermal conductivity and hardenability of the steel are inversely proportional, and it was difficult to obtain a steel having excellent thermal conductivity and hardenability at the same time. However, the present invention provides 0.37 to 0.46 wt% and 0.0007 to 0.004 wt% of carbon and boron, respectively. By limiting to%, it is possible to provide a hot die steel having excellent thermal conductivity and hardenability at high temperature.

On the other hand, the manufacturing method of hot die steel according to another embodiment of the present invention, 0.37 to 0.46% by weight of carbon (C), 0.25 to 0.5% by weight of silicon (Si), 0.36 to 0.56 weight of manganese (Mn) %, Chromium (Cr) 2.0 to 5.0 wt%, Molybdenum (Mo) 1.4 to 2.6 wt%, Vanadium (V) 0.4 to 0.8 wt%, Boron (B) 0.0007 to 0.004 wt%, Aluminum (Al) 0.002 to 0.022 wt% %, 0.001 to 0.09 weight percent of titanium (Ti), the remainder is a forging step of heat-treating a hot mold ingot containing Fe and unavoidable impurities; Quenching step of heating and then cooling the mold material produced in the forging step; And a tempering step of heat-treating the mold material quenched in the quenching step.

In this case, when the content value of the elements forming the hot die steel is substituted into Equation (1), the value is 30.5 or more, and when it is substituted into Equation (2), the value is 0.35 or less.

In addition, the hot mold ingot may further include 0.001 to 0.007% by weight of tungsten, 0.001 to 0.025% by weight of niobium, and 0.005 to 0.022% by weight of cobalt. Examples of inevitable impurities include phosphorous, sulfur, and nitrogen. But it is not limited thereto. In addition, since the effect obtained by limiting the type and content of the elements forming the hot die ingot, the upper and lower limits of the content thereof is the same as the hot die steel described above, the description thereof is omitted.

The method for manufacturing hot die steel first melts the metal using any one of an artificial heat source, for example, an electric furnace, a vacuum induction furnace and an atmospheric induction furnace, and then generates oxygen, hydrogen, It can be prepared by removing a gas such as nitrogen.

Next, the forging step of heat-treating the hot die ingot in the temperature range of 850 ~ 1300 ℃ is carried out, the casting structure of the hot die ingot is destroyed through the forging step, and the pores inside the hot die ingot generated during solidification By removing, the internal quality of the hot mold ingot can be improved, and at this time, the shape as the mold material can be formed.

In this case, if the temperature is less than 850 ° C, cracks may occur due to difficulty in deformation of the shape during forging, and if the temperature exceeds 1300 ° C, cracks may be generated due to high temperature embrittlement due to overheating. It is preferable to carry out.

In addition, the forging ratio in the forging process is preferably 4.5S or more. By forging the hot die ingot at such forging ratio, the efficiency of compressing and removing pores present in the hot die ingot increases, so that the structure of the hot die steel is more finely formed. Can be. If the forging ratio is less than 4.5S, because the structure of the mold steel is coarse, the toughness is weakened, so that the quality of the produced product may deteriorate when applied to die casting, it is preferable that the forging step is performed with the forging ratio to 4.5S or more.

After the spheroidization heat treatment step may be performed, the spheroidization heat treatment step may be spheroidized by splicing the network carbide formed in the microstructure of the mold material due to the forging process to uniformize the carbon content to improve efficiency in the quenching step to be described later and finally hot The step of improving the strength and hardness of the mold ingot, wherein if the heat treatment temperature is carried out below 840 ℃, the decomposition of the network carbide is not sufficiently performed so that the quenching efficiency increase is small, when the heat treatment temperature exceeds 900 ℃ spheroidization heat treatment step Since alloy carbides produced through coarsening may be difficult to obtain the desired properties after the quenching step, the spheroidizing heat treatment step is preferably performed at a temperature range of 840 ~ 900 ℃.

Next, the quenching step is performed, and the quenching is performed after the heat treatment and cooling. When the heat treatment temperature in the quenching step is less than 1000 ° C., the hardening effect of the added alloying element may be low, and thus the hardenability may be reduced. If the temperature is exceeded, the amount of retained austenite is increased by lowering the martensite transformation start temperature due to the coarsening of the particles, resulting in deterioration of mechanical properties and material irregularities, which may cause mold change. Therefore, the heat treatment is preferably performed at a temperature range of 1000 ~ 1040 ℃.

The cooling process performed after the heat treatment process of the quenching step may be performed, wherein the cooling is carried out to reach a temperature range of 80 to 100 ℃ at a rate of 0.35 ℃ / s or more, preferably 0.5 ~ 3.0 ℃ / s By doing so, the strength of the mold steel can be further improved. At this time, the cooling rate may be achieved by cooling using a high pressure nitrogen pressurized cooler.

The tempering step is performed after the quenching step, and the heat treatment temperature at this time is preferably 520 to 640 ° C. When the tempering temperature is less than 520 ° C., the secondary curing reaction does not sufficiently occur to obtain the desired physical properties. Tempering brittleness due to carbides generated during secondary curing occurs, and if the strength of the mold steel is sharply lowered when it exceeds 640 ° C., the tempering step is preferably performed within the above temperature range.

Tempering is a process carried out to improve the toughness of the hot die steel, preferably in the present invention is carried out in multiple stages, specifically, after the first tempering for 2 to 6 hours in the temperature range of 540 ~ 630 ℃, 540 ~ The second tempering may be included for 2 to 6 hours in the temperature range of 620 ℃.

Thereafter, a third tempering step of heat-treating the hot mold steel obtained through the secondary tempering for 2 to 6 hours at a temperature range of 540 to 610 ° C. may be further included.

In the multi-stage tempering as described above, residual austenite in the mold steel structure is decomposed into bainite or transformed into martensite by primary tempering, thereby decreasing toughness. By increasing and tertiary tempering, the hardness of the mold material can be precisely controlled.

Thereafter, a surface nitriding heat treatment step may be further performed to improve the surface hardness of the hot die steel manufactured by the above-described method, and the nitriding heat treatment may be performed on the steel at about 500 ° C. or higher at about 500 ° C. or higher under ammonia gas. Pure nitriding method by heating for natural cooling, gas nitriding method by CO gas supplied from nitrogen element and carburizing gas due to pyrolysis of ammonia, alkali metal cyanate (MCNO) decomposition over 500 ℃ The soft nitriding method to be used, the ion nitriding method which causes the N + ion generated by ionizing nitrogen gas by the discharge energy to nitriding on the steel surface of the negative electrode, and the immersion nitriding method using plasma may be used, but are not limited thereto. .

Hereinafter, the present invention will be described in more detail with reference to specific examples. The following examples are merely examples to help understanding of the present invention, but the scope of the present invention is not limited thereto.

[Production example]

The steel ingot having the composition shown in Table 1 was forged at a forging ratio of 5S at about 1185 ° C. to prepare a mold material, and a spheroidizing heat treatment was performed at 840 ° C. for about 10 hours. Thereafter, the mold material is heat-treated at 1030 ° C. for 2 hours, subjected to a quenching process of cooling at a cooling rate of about 0.5 ° C./s to about 90 ° C., and then first tempered at 595 ° C. for 3 hours, and then at 590 ° C. After tempering for 2 hours for 3 hours and then tempering for 3 hours at 580 ° C., hot mold steels of Examples 1 to 3 and Comparative Examples 1 to 8 were prepared.

C Si Mn Cr Mo V B Al Ti W Nb Co Ni Example 1 0.38 0.49 0.45 4.65 1.44 0.626 0.0027 0.003 0.001 0.003 0.003 0.011 0.11 Example 2 0.42 0.48 0.44 4.95 1.41 0.580 0.0010 0.002 0.001 0.006 0.001 0.005 0.08 Example 3 0.37 0.31 0.48 2.92 1.86 0.590 0.0019 0.019 0.004 0.000 0.023 0.019 0.09 Comparative Example 1 0.45 0.30 0.46 4.90 1.27 0.529 0.0003 0.000 0.000 0.000 0.000 0.000 0.12 Comparative Example 2 0.39 0.32 0.45 2.93 1.46 0.590 0.0008 0.037 0.000 0.000 0.000 0.000 0.05 Comparative Example 3 0.40 0.51 1.17 1.00 2.5 1.210 0.0016 0.024 0.000 0.000 0.000 0.000 0.10 Comparative Example 4 0.41 0.26 0.40 2.04 2.58 0.410 0.0007 0.055 0.090 0.000 0.000 0.000 1.02 Comparative Example 5 0.38 0.91 0.42 5.16 1.22 0.850 0.0001 0.011 0.001 0.003 0.003 0.015 0.05 Comparative Example 6 0.38 0.31 0.45 4.87 1.17 0.590 0.0009 0.048 0.020 0.000 0.025 0.000 0.10 Comparative Example 7 0.37 1.00 0.25 5.00 1.25 1.000 0.0000 0.000 0.000 0.000 0.000 0.000 0.00 Comparative Example 8 0.41 0.44 0.42 3.96 1.43 0.700 0.0010 0.005 0.003 0.000 0.000 0.000 0.00

(Unit: wt%)

Experimental Example 1 Simulation of Thermal Conductivity and Critical Cooling Rate According to Carbon and Boron Content

The thermal conductivity represented by Equation (1) is simulated at 400 ° C. by using computer simulation and statistical tools, and the results are shown in FIG. 2 and the critical cooling rate expressed by Equation (2). The log value of is simulated and the result is shown in FIG. At this time, the contents of silicon, manganese, chromium, molybdenum, titanium and vanadium were fixed at 0.45, 0.46, 4.8, 1.45, 0.001 and 0.6 wt%, respectively.

Referring to FIG. 2, when the carbon content is 0.37 wt% or more, the thermal conductivity calculated by Equation (1) may be 30.5W / mK or more, indicating that the heat check resistance of the mold steel is excellent. 3, when the boron content is 0.0007 wt% or more, the critical cooling rate is 0.35 ° C./s or less, that is, the log value of the critical cooling rate is −0.45 or less, indicating that the mold hardening ability is excellent.

Therefore, when the mold steel of the present invention contains 0.37% by weight or more of carbon and at the same time contains boron of 0.0007% by weight or more, it can be seen that the thermal conductivity and the hardenability are excellent, unlike the existing mold steel.

Experimental Example 2 Measurement of Thermal Conductivity and Critical Cooling Rate

The thermal conductivity and critical cooling rate of the hot die steels of Examples 1 to 3 and Comparative Examples 1 to 8 were calculated using the same method as in Experimental Example 1, and Table 2 shows the thermal conductivity and critical cooling rate of some hot mold steels. It is shown in Table 2 together.

Thermal conductivity measurement was performed for Examples 1, Comparative Examples 1 to 5 and 7, and the thermal conductivity was calculated by measuring the density, specific heat and thermal diffusivity, the density was measured by the submerged method, specific heat and thermal diffusivity Was measured using the Laser flash method.

In addition, the critical cooling rate measurement was performed for Example 2, Comparative Examples 1 and 2, using the dilatometer to derive the continuous cooling transformation diagram of Figure 4, using this to minimize the transformation does not occur during cooling The cooling rate was determined by the critical cooling rate.

division Thermal conductivity
Measured value (W / mK) Value of equation (1)
(W / mK) Derived from a continuous cooling transformation diagram
Critical cooling rate (℃ / s) Value of equation (2)
(℃ / s) Example 1 31.7 30.6 - 0.09 Example 2 - 30.9 0.20 0.18 Example 3 - 33.1 - 0.35 Comparative Example 1 31.8 32.0 0.50 0.59 Comparative Example 2 33.8 33.5 1.25 1.44 Comparative Example 3 33.3 32.7 - 0.40 Comparative Example 4 34.4 35.1 - 0.79 Comparative Example 5 28.2 28.6 - 0.52 Comparative Example 6 - 31.2 - 0.45 Comparative Example 7 28.2 28.4 - 0.80 Comparative Example 8 - 31.9 - 0.47

Experimental results show that the thermal conductivity simulation value using Equation (1) and the actual measured value are somewhat different, but the tendency is exactly the same, confirming that Equation (1) can be used as an indicator of thermal conductivity. Similarly, the critical cooling rate using Equation (2) and the critical cooling rate derived from the continuous cooling transformation diagram are somewhat different in absolute value, but the tendency is the same, and Equation (2) is an index indicating the critical cooling rate. It was confirmed that it can be utilized.

Therefore, the value calculated by Equation (1) was used to replace the thermal conductivity, and the value calculated by Equation (2) was used by replacing the critical cooling rate.

The thermal conductivity values using Equation (1) were all higher than 30.5W / mK except for the mold steels of Comparative Examples 5 and 7, but the critical cooling rate using Equation (2) was 0.35 ° C only in Examples 1 to 3. It was confirmed that the mold steels of Examples shown below / s satisfy all of the conditions of the formulas (1) and (2), and have high thermal conductivity and excellent hardenability.

Experimental Example 3 Physical Property Evaluation

The results of measuring the tensile strength and impact toughness of the hot die steels of Examples 2 and Comparative Examples 1, 2, and 4 were made into 300 × 300 × 300 mm, and the test pieces were taken in the forging length direction from the surface areas. Indicated. At this time, the hardness was measured before measuring the physical properties, the results are also shown in Table 3. Tensile strength was obtained by testing the standard of ASTM E8 at room temperature, and the impact toughness was obtained by Charpy impact test (2mm U-notch) at the standard of ASTM E23 at room temperature.

In formula (2)
Value (℃ / s) Tensile Strength (Mpa)
(Hardness (HRc)) Impact Toughness (J)
(Hardness (HRc)) Example 2 0.18 1566 (45.7) 29.7 (49.6) Comparative Example 1 0.59 1548 (45.5) 25.0 (49.7) Comparative Example 2 1.44 1487 (45.8) 8.8 (49.1) Comparative Example 4 0.79 1492 (45.6) 5.2 (49.2)

As confirmed in the results of Table 3, the tensile strength of Example 2 was found to be the most excellent at a hardness of about 46HRc, the impact toughness of Example 2 was found to be excellent at the same hardness.

This result appears because Example 2 satisfies the composition defined in the present invention and at the same time satisfies Equation (2). Particularly, it satisfies Equation (2), thereby ensuring a low critical cooling rate, thereby improving hardenability. Therefore, it can be seen that the tensile strength and impact toughness are improved.

Therefore, in order to improve the tensile strength and impact toughness of the hot die steel in the alloy system of the present invention, it is preferable to satisfy both the composition and the formula (2) defined in the present invention.

Experimental Example 4 Observation of Tissue Change and Hardness According to Quenching Temperature

The quenching temperature of the mold steel of Example 1 was changed, and the microstructure change according to the quenching temperature was observed using the Debye ring photograph and the optical photograph shown in FIG. 6. Hardness was measured and shown in FIG. 7.

5 is an X-ray diffraction image of the Debye ring of martensite, the matrix structure of the mold steel, and the Debye ring has a sharp shape when the quenching temperature is 1050 ° C, which is a result of abnormal grain growth of grains having a specific orientation. Judging. In addition, since the debye ring of the retained austenite is also seen at 1050 ° C. or higher, when the quenching temperature is 1050 ° C. or higher, the tissue is formed inhomogeneously. Such a result can be confirmed in FIG. It can be seen that the quenching temperature is preferably less than 1050 ° C.

In general, as the quenching temperature increases, the solubility of the alloying elements increases to improve the hardenability. As can be seen in FIG. 7, the hardness rapidly increases around 1000 ° C., and then the hardness drops rapidly around 1040 ° C., thereby reducing the hardness of the mold steel. It can be seen that quenching heat treatment is preferably performed at 1000 ~ 1040 ℃ to improve the hardness.

Experimental Example 5 Measurement of Hardness According to Tempering Temperature

According to the preparation example, the mold steel having the composition of Examples 1, 2 and Comparative Example 1 was prepared, but the two-stage tempering process was performed while varying the temperature in the temperature range of 300 ~ 700 ℃, the results are shown in FIG. Indicated. At this time, the temperature of each tempering step was adjusted to be the same.

Referring to FIG. 8, when the tempering temperature is 540 ° C. or less, the hardness on the graph is high, but there is a risk of temper brittleness due to secondary curing, and when the temperature is exceeded 630 ° C., severe hardness decreases. You can check it. Therefore, the tempering process is preferably carried out at a temperature range of 540 ~ 630 ℃, it was confirmed that a high hardness of 45HRc or more can be obtained at this time. In addition, it can also be seen that the hardness of the example is relatively higher than the hardness of the comparative example in the above temperature range.

Experimental Example 6 Softening Resistance Evaluation

In order to evaluate the change in hardness according to the tempering time, hot die steels of Example 1 and Comparative Examples 1, 2, 5, and 7 were prepared in the same manner as in Preparation Example, but the tempering temperature was 650 ° C., 0.01, 1, After heat treatment for 2, 5, 10, 20, 30, 40, 50 and 100 hours and then cooled, the hardness of each specimen was measured, and the results are shown in FIG. 9.

As a result, the initial hardness was similar to each mold steel, but the least decrease in hardness after 100 hours was found to be the mold steel of Example 1. The softening resistance, which is the resistance to hardness decrease at high temperatures, is directly proportional to the amount of molybdenum, titanium, and niobium forming stable carbonitrides, which also showed high softening resistance of Comparative Example 2 having high molybdenum content. We can confirm from thing, too.

Therefore, in order to improve the softening resistance of hot die steel, it is preferable that molybdenum is contained in 1.4 weight% or more.

Experimental Example 7 Evaluation of Nitriding Properties

After the surface of the hot die steel of Example 1, Comparative Examples 1 and 5 by using the gas immersion nitriding method at 550 ℃ for 15 hours, the hardness according to the distance from the surface was measured, the results are shown in Figure 10 .

As a result, in Example 1, the surface hardness after nitriding was increased by 701 Hv relative to the hardness of the base material portion, in Comparative Example 1 was increased by 425Hv, and Comparative Example 5 was increased by 553Hv, and the surface hardness of Example 1 was higher than that of the comparative examples. It can be seen that the improvement is more than 25%.

This is a result of the hot die steel of the present invention effectively delays the propagation of the heat check crack, and through this, it can be predicted that the life of the die manufactured with the hot die steel of the present invention will be much superior to other die steels. .

The increase in hardness due to nitriding can be achieved through molybdenum, chromium and vanadium, which is mainly caused by molybdenum, and especially when the molybdenum content is less than 1.4 wt%, the rise in hardness is significantly lowered. It can be seen that it is preferable to use.

Experimental Example 8 Heat Check Evaluation

After heating the mold steel specimens of Examples 2 and 3 and Comparative Examples 1, 5 to 7, and 8 to 650 ° C by using a high frequency induction heating method, the cycle of cooling to room temperature through water cooling was repeated 1000 times. The number and length of the generated heat cracks were measured, and the results are shown in Table 4. In addition, the thermal conductivity of each specimen was calculated according to Equation (1) and is shown in Table 4, and the maximum heat crack length and heat crack length per unit specimen length according to thermal conductivity are shown in FIG. 11.

division Value of Equation (1) (W / mK) A: average heat crack length
(Μm) B: Number of heat cracks per unit specimen length (mm ^-1 ) Heat crack length (mm) A x B: Average heat crack length per unit specimen length (㎛ / mm) Example 2 30.9 215 2.37 1.96 509.55 Example 3 33.1 181 2.25 1.16 407.25 Comparative Example 1 32.0 292 2.39 2.24 697.88 Comparative Example 5 28.6 349 2.24 2.33 781.76 Comparative Example 6 31.2 257 2.35 2.07 603.95 Comparative Example 7 28.4 460 1.99 3.78 915.40 Comparative Example 8 31.9 321 2.12 2.31 680.52

As a result of the experiment, the number of heat cracks per unit specimen length was similar to the comparative example, but the average heat crack length was found to be significantly shorter, and the heat crack length per unit specimen length was significantly lower than that of the comparative example. Since it was confirmed that the heat cracking resistance of the mold steel was excellent, it was confirmed that by using the mold steel of the present invention, a mold having an extended life than the conventional mold steel could be obtained.

In addition, referring to FIG. 11, it can be seen that as the thermal conductivity increases, the maximum heat crack length, the average heat crack length x the number of heat cracks per unit specimen length decrease. In particular, since the heat crack resistance is significantly improved when the value of the thermal conductivity calculated by Equation (1) is 30.5W / mK or more, it is preferable that the thermal conductivity value of the hot die steel calculated by Equation (1) is 30.5W / mK or more. .

At this time, the thermal conductivity of Comparative Examples 1, 6, and 8 is 30.5W / mK or more, and the heat crack resistance was superior to Comparative Examples 5 and 7, but it was found to be significantly lower than Examples 2 and 3. This suggests that the condition of the composition and the formula (2) defined in the present invention as well as the formula (1) in order to improve the heat crack resistance is to be satisfied, the details can be confirmed in Experimental Example 10 to be described later.

Experimental Example 9 Yongson Evaluation

The mold steels of Examples 2 and 3 and Comparative Examples 1, 5, and 7 were made into 20 × 20 × 10 mm specimens, and then a viscous refractory material was applied to only one surface of each specimen and sufficiently dried for at least 48 hours. Thereafter, the specimen was immersed in a molten aluminum having a temperature of 700 ° C. for 43 hours, then taken out and cooled, and then cut in the vertical direction of the refractory coating surface to take a cross section using an optical microscope, and the results are shown in FIG. 12. .

Thereafter, the average value of the depth of the molten iron was measured 10 times from the point where the molten iron was not formed because the refractory material was not applied. The heat crack per unit specimen length obtained in Experimental Example 8 was shown in FIG. 13. The X-axis of the graph was composed of molybdenum content, and the melt loss values of Comparative Examples 5, 7, 1, and Examples 2 and 3 were in the order of low to high molybdenum content, and square dots of each data For the melting depth, the circular dot represents the length of the heat crack per unit specimen length.

Referring to Figure 12, the result of the evaluation of the melting loss of the comparative example was shown to be more than 270㎛ depth of the melting loss of the embodiment is less than about 230㎛ it can be seen that the melting property of the embodiment is superior to the comparative example, and looking at Figure 13 It can be seen that the melting property is proportional to the content of molybdenum.

Through the above experiment, the hot mold steel of the present invention can increase the amount of stable carbide produced during the secondary hardening of the mold steel by limiting the molybdenum content to more than 1.4% by weight and can evenly distribute them to improve the melt loss resistance It was confirmed that there is.

Experimental Example 10

In order to confirm that the performance of the alloy is improved when all of the limited alloy composition, Equation (1) and Equation (2) are satisfied in the present invention, an embodiment corresponding to the case where all three conditions are not satisfied is satisfied. And for the Comparative Example specimens, the results of measuring the average heat crack length and the number of heat cracks per unit length of the specimen are summarized in Table 5. In this case, if the composition, Equation (1) and Equation (2) are satisfied, 'O' is indicated, otherwise 'X' is indicated.

division Furtherance Equation
(One) Equation
(2) A: average heat crack length (µm) B: unit specimen
Number of heat cracks per length (mm ^-1 ) maximum
Heat Crack
Length (mm) A x B: Unit Specimen
Average per length
Heat Crack Length
(Μm / mm) Example 2 O O O 215 2.37 1.96 509.55 Example 3 O O O 181 2.25 1.16 407.25 Comparative Example 1 X O X 292 2.39 2.24 697.88 Comparative Example 5 X X X 349 2.24 2.33 781.76 Comparative Example 6 X O X 257 2.35 2.07 603.95 Comparative Example 7 X X X 460 1.99 3.78 915.40 Comparative Example 8 O O X 321 2.12 2.31 680.52

As a result of the measurement, it was confirmed that the heat crack resistance of the hot die steel that does not satisfy at least one of the composition of the hot die steel, the formula (1) and the formula (2) is significantly lowered.

In particular, when comparing the embodiments that satisfy all the conditions and the comparative examples that do not satisfy the one or more conditions, it can be seen that the embodiment is improved heat crack resistance by at least 20%, up to 120% than the comparative example.

That is, in the case of hot die steel containing carbon, silicon, manganese, chromium, molybdenum, titanium, vanadium, boron and aluminum as alloying elements, the formula (1) related to the composition, thermal conductivity defined in the present invention, and the formula related to hardenability It can be seen that the heat crack resistance is greatly improved when all of the conditions (2) are satisfied.

Therefore, when all of the conditions of the composition, formula (1) and formula (2) defined in the present invention are satisfied, the hot die steel may have improved heat crack resistance and mechanical properties, and is manufactured using such hot die steel. The mold can have a long life and at the same time exhibit improved performance as a mold.

In addition, since such hot die steel exhibits excellent nitriding properties, it is expected that the additional nitriding heat treatment will have further improved heat crack resistance and mechanical properties.

The present invention is not limited to the above specific embodiments and descriptions, and various modifications can be made by those skilled in the art without departing from the gist of the invention as claimed in the claims. Such variations are within the protection scope of the present invention.

Claims (21)

0.37 to 0.46% by weight of carbon (C), 0.25 to 0.5% by weight of silicon (Si), 0.36 to 0.56% by weight of manganese (Mn), 2.0 to 5.0% by weight of chromium (Cr), molybdenum (Mo) 1.4 ~ 2.6 wt%, Vanadium (V) 0.4 ~ 0.8 wt%, Boron (B) 0.0007 ~ 0.004 wt%, Aluminum (Al) 0.002 ~ 0.022 wt%, Titanium (Ti) 0.001 ~ 0.09 wt%, Tungsten (W) 0.001 To 0.007% by weight, niobium (Nb) 0.001 to 0.025% by weight, cobalt (Co) 0.005 to 0.022% by weight and iron (Fe) and unavoidable impurities,
The weight% value of each element included satisfies the following formulas (1) and (2),
Obtained through forging step, quenching step, spheroidizing heat treatment step, tempering step and nitriding heat treatment step,
The forging step is in the temperature range of 850 ~ 1300 ℃, the quenching step in the temperature range of 1000 ~ 1040 ℃, the spheroidizing heat treatment step in the temperature range of 840 ~ 900 ℃, the tempering step is the temperature of 520 ~ 640 ℃ Performed in a range,
The tempering step is carried out in a multi-stage tempering method performed at least two times,
Hot die steel, characterized in that the die-casting die steel.

delete delete delete delete delete 0.37 to 0.46% by weight of carbon (C), 0.25 to 0.5% by weight of silicon (Si), 0.36 to 0.56% by weight of manganese (Mn), 2.0 to 5.0% by weight of chromium (Cr), molybdenum (Mo) 1.4 ~ 2.6 wt%, Vanadium (V) 0.4 ~ 0.8 wt%, Boron (B) 0.0007 ~ 0.004 wt%, Aluminum (Al) 0.002 ~ 0.022 wt%, Titanium (Ti) 0.001 ~ 0.09 wt%, Tungsten (W) 0.001 ~ 0.007 wt%, niobium (Nb) 0.001 to 0.025 wt%, cobalt (Co) 0.005 to 0.022 wt%, the remainder of the hot mold ingots in the temperature range of 850 ~ 1300 ℃, containing iron (Fe) and unavoidable impurities Forging step of heat treatment;
Quenching step of heating the mold material produced in the forging step in the temperature range of 1000 ~ 1040 ℃ and then cooling;
Spheroidizing heat treatment step of heat-treating the mold material obtained through the quenching step in a temperature range of 840 ~ 900 ℃;
Tempering step of heat-treating the mold material obtained through the spheroidizing heat treatment step at a temperature range of 520 ~ 640 ℃; And
Nitride heat treatment step of heat-treating the mold material obtained through the tempering step in the presence of nitrogen or nitrogen compounds;
The tempering step is carried out in a multi-stage tempering method performed at least two times,
A method for producing hot die steel, wherein the weight% value of each element included satisfies the following formulas (1) and (2).

delete The method of claim 7, wherein
The forging step is a manufacturing method of hot die steel, characterized in that performed at a forging ratio of 4.5S or more.
delete delete delete The method of claim 7, wherein
The cooling process of the quenching step is a method of manufacturing hot die steel, characterized in that carried out at a cooling rate of 0.35 ℃ / s or more.
The method of claim 7, wherein
The cooling process of the quenching step is a method of manufacturing hot die steel, characterized in that cooling to a temperature range of 80 ~ 100 ℃.
The method of claim 7, wherein
The tempering step, the first tempering step of heat-treating the quenched mold material for 2 hours to 6 hours at a temperature range of 540 ~ 630 ℃; And
A secondary tempering step of heat-treating the mold material for 2 hours to 6 hours in a temperature range of 540 to 620 ° C; Method of manufacturing hot mold steel comprising a.
The method of claim 15,
And a third tempering step of heat-treating the hot mold steel obtained through the second tempering step for 2 hours to 6 hours at a temperature range of 540 to 610 ° C .;
delete The method of claim 7, wherein
The nitriding heat treatment step is a method of manufacturing a hot die steel, which is carried out by any one of the method of pure nitriding, gas nitriding, soft nitriding, ion nitriding and distillation nitriding.
delete delete delete

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